The technology of the present disclosure relates generally to distributed antenna systems (DASs) for distributing communications services to remote areas each forming a coverage area and particularly to frequency independent isolation of duplexed ports in DASs.
A cellular communications system can be provided that includes cellular base stations configured to communicate with cellular client devices to provide analog cellular communications services. These cellular base stations are typically co-located with cellular antennas configured to distribute transmitted wireless cellular communications signals from a cellular base station to cellular client devices residing within the wireless range of a cellular antenna. The cellular antennas are also configured to receive transmitted wireless cellular communications signals from cellular client devices to the cellular base station for transmission over a cellular network.
It may be desired to distribute cellular communications services remotely, such as in a building or other facility, to provide clients access to such cellular communications services within the building or facility. One approach to distributing cellular communications services in a building or facility involves use of radio frequency (RF) antenna coverage areas, also referred to as “antenna coverage areas.” The antenna coverage areas can have a radius in the range from a few meters up to twenty meters, as an example. Combining a number of access point devices creates an array of antenna coverage areas. Because the antenna coverage areas each cover small areas, there are typically only a few users (clients) per antenna coverage area. This allows for minimizing the amount of RF bandwidth shared among the wireless system users.
As an example, FIG. 1 illustrates distribution of communications services to remote coverage areas 10 of a DAS 12. In this regard, the remote coverage areas 10 are created by and centered on remote antenna units 14 connected to a head-end equipment 16 (e.g., a head-end controller or head-end unit). The head-end equipment 16 may be communicatively coupled to a base station 18. In this regard, the head-end equipment 16 receives downlink communications signals 20D from the cellular base station 18 to be distributed to the remote antenna units 14. The remote antenna units 14 are configured to receive downlink communications signals 20D from the head-end equipment 16 over a communications medium 22 to be distributed to the coverage areas 10 of the remote antenna units 14. Each remote antenna unit 14 may include an RF transmitter/receiver (not shown) and an antenna 24 operably connected to the RF transmitter/receiver to wirelessly distribute the cellular services to client devices 26 within the coverage area 10. The remote antenna units 14 are also configured to receive uplink communications signals 20U from the client devices 26 in the coverage area 10 to be distributed to the cellular base station 18. The size of a given coverage area 10 is determined by the amount of RF power transmitted by the remote antenna unit 14, the receiver sensitivity, antenna gain and the RF environment, as well as by the RF transmitter/receiver sensitivity of the cellular client device 26. Cellular client devices 26 usually have a fixed RF receiver sensitivity, so that the above-mentioned properties of the remote antenna unit 14 mainly determine the size of the remote coverage areas 10.
The equipment in the DAS 12 in FIG. 1 may be provided to support wide radio bands of spectrum commonly used in the cellular industry. For example, a personal communications services (PCS) band may be supported by the DAS 12 that includes the 1850-1910 MegaHertz (MHz) radio band for uplink signals and 1930-1990 MHz band for downlink signals. A cellular radio band may also be supported by the DAS 12 that includes the 824-859 MHz radio band for uplink signals and the 869-894 MHz band for downlink communications signals. In this regard, it may be required to couple a base station to a DAS, such as DAS 12 in FIG. 1, through a duplexed port. A duplexed port allows a DAS to simultaneously receive downlink signals into the DAS and transmit uplink communications signals from the DAS.
In this regard, FIG. 2 illustrates exemplary downlink and uplink path circuits 28D, 28U provided in respective downlink and uplink communications paths 30D, 30U in the DAS 12 of FIG. 1. The downlink and uplink communications paths 30D, 30U extend between the base station 18 and the remote antenna unit 14. The base station 18 is coupled to the DAS via a duplexed port 32. The duplexed port 32 receives downlink communications signals 20D from the base station 18 to be provided to the DAS 12 via the HEE 16 in this example. The duplexed port 32 also receives uplink communications signals 20U from the DAS 12 via the HEE 16 to be provided to the base station 18. A head-end duplexer 34(H) is provided in the HEE 16. The head-end duplexer 34(H) is coupled to the duplexed port 32. The head-end duplexer 34(H) is configured to separate a duplexed downlink and uplink communications path 36 into the separate downlink communications path 30D and a separate uplink communications path 30U. Downlink communications signals 20D are coupled from the head-end duplexer 34(H) to the head-end downlink circuits 28D(H). The downlink communications signals 20D are then distributed from the head-end downlink circuits 28D(H) to the remote downlink circuits 28D(R) in a remote antenna unit 14 to be transmitted through the antenna 24 of the remote antenna unit 14. The uplink communications signals 20U are coupled from the antenna 24 of the remote antenna unit 14 to a remote duplexer 34(R), and from the remote duplexer 34(R) to the remote uplink circuits 28U(R). The uplink communications signals 20U are distributed to the head-end uplink circuits 28U(H), and from the head-end duplexer 34(H) to the base station duplexed port 38.
With continuing reference to FIG. 2, due to expansion of radio bands, the frequency gap between downlink communications signals 20D and the uplink communications signals 20U supported in the DAS 12 may become smaller. For example, the frequency gap between the downlink communications signals 20D and the uplink communications signals 20U may be 10 MHz or less. If frequency gap between the downlink communications signals 20D and the uplink communications signals 20U is too small, it may be difficult or even impossible to provide the required isolation between the downlink and uplink communications paths 30D, 30U in the head-end duplexer 34(H) while maintaining other requirements of the head-end duplexer 34(H), such as low attenuation, lower ripple (i.e., variance in frequency response), small size, and/or low cost. If the isolation provided by head-end duplexer 34(H) is lower than required, a portion of the uplink communications signal 20U can leak through the head-end duplexer 34(H) to the downlink communications path 30D, as shown by leakage path 40 in FIG. 2. This leakage through the downlink communications path 30D might distort the downlink communications signal 20D or even create oscillations on the downlink communications signal 20D.